Conventional video cameras are image scanning devices. A video signal is acquired by imaging an object scene to a remote surface and converting the illuminance at that surface to an electrical signal. For vidicons, the conversion is made by scanning an electron beam over a photoconductive surface.
Manufacturing processes are finding increased utility for such video systems. The obvious use has been for remote observation, where an operator is aided by a camera and monitor in examining a part for alignment, measurement or inspection. More recently, video signals have been processed electronically for automatic recognition of image content, thereby eliminating subjective human observation and judgment. Advantages of increased speed, adaptability and reliability are realized with such automatic image processing, and its use for manufacturing processes is expected to increase in the future.
In most cases, recognition of image content is made by noting the variations in the video signal level. Large variations are obviously desirable, particularly in instances where automatic image processing is to be performed. Signal levels representing different image detail are often separated with thresholding circuitry to produce binary images (0 or 1); in this form, automatic image processing is most readily performed. This necessarily implies the need for a high contrast image or a change in video gain to increase the video signal variations. However, in many instances, the contrast is low, making the transformation to a binary image difficult or impossible.
One technique that has been used to improve the contrast of a video signal is described in an article titled "Laser TV Camera Systems" in The Proceedings of SPSE Seminar, 1968, by R. S. Rowley. That article describes a system in which a laser beam is raster scanned over the surface of an object and a detector monitors the light reflected from the object. The detected reflections are converted into a video signal which is amplified and presented directly to the video input of a TV for display. Such an arrangement is referred to as object scanning and eliminates the requirement of a video camera and imaging optics. Although such a technique can be most effective when outside illumination is poor or non-existent, the video signal contrast is improved only slightly compared to that produced with image scanning devices.
Accordingly, there is a need for a technique for obtaining an enhanced video signal which increases the video contrast and is amenable to automatic recognition of image content in order to eliminate subjective human observation and judgment.